1. Field of Invention
The present invention is directed to thermoset polymer toughening agents and to vinyl ester and unsaturated polyester resins toughened using these toughening agents.
2. Brief Description of the Prior Art
Vinyl ester (VE) resins are one of the most popular resin systems used in polymer matrix composite fabrication for military and commercial applications due to their good properties, low weight and low cost. VE resins typically contain two components, dimethacrylate esters and a reactive diluent. The dimethacrylate esters are commonly made by, for example, reacting methacrylic acid with bisphenol A-based epoxy resin. A typical reactive diluent is styrene, in an amount of 40-60 wt % to provide processing ease and chemical reactivity. These VE resin systems normally possess low viscosity (<500 cP) at room temperature and can be readily processed using low cost tooling such as vacuum assisted resin transfer molding (VARTM).
VE resins have superior properties relative to unsaturated polyester (UPE) systems and are less expensive and easier to process than epoxy systems. However, like other thermoset resins, unmodified vinyl esters suffer from brittleness and low resistance to fracture that limits their application as structural materials. As a result, significant effort has been expended on toughening VE resins. Common toughening methods include: 1) increasing the molecular weight of the VE resins, 2) incorporating a chain extender into the VE resin network, and 3) introducing additives to the VE resin system as toughening agents. Typical toughening agents may include rigid thermoplastics or rubber particles. Among these three options, the use of a toughening agent to form a second dispersed phase in the VE resin provides a desired means of improving fracture toughness since this option provides both the desired toughening effect and an acceptable level of change in the glass transition temperature (Tg) of the resin.
The most frequently used toughening modifiers for VE resins are liquid rubbers that are based on butadiene-acrylonitrile copolymers terminated with various functionalities like vinyl, epoxy and carboxyl. Auad M. L., et al., Polymer 2001, 42:3723-3730; Robinette E. J. and Palmese G. R., Polymer 2004, 43:6143-6154; Dreerman E., et al., Journal of Applied Polymer Science, 1999, 72:647-657; Ullett J. S., Polymer Engineering and Science, 1995, 35:1086-1097; Auad M. L., et al., Journal of Applied Polymer Science, 2003, 89:274-283 and Auad M. L., et al., Journal of Materials Science, 2002, 37:4117-4126. These liquid rubbers are low-molecular-weight elastomer prepolymers and can mix with VE resins to form a stable, homogeneous system at room temperature in the presence of significant amounts of styrene or under mechanical agitation due to their relative immiscibility with VE resins. It is difficult to form a miscible system with epoxy derived VE resins using liquid rubbers with molecular weights ranging from 3000 g/mol to 4000 g/mol unless more polar groups are contained in the liquid rubber, as is the case for high acrylonitrile content (26%) carboxyl terminated butadiene-acrylonitrile copolymers (CTBN). Auad M. L., et al., Polymer 2001, 42:3723-3730. This approach is, nevertheless, insufficient for the high molecular weight VE resins with low solubility parameters, especially the recently developed novolac VE resins with multi-functionality and low VOC VE resins with styrene partially replaced by environmental friendly diluents like methacrylated fatty acid (MFA). La Scala J. J. et al., Polymer 2005, 46:2908-2921 and Geng, X., et al., Composites Research Journal 2009; 2:36-42.
Grafting liquid rubber directly to the VE resin backbone is an effective method to improve the compatibility of toughening modifiers with VE resin. For example, Derakane™ 8084 is a toughened version of Derakane™ 411-45 in which about 7% of a low-molecular-weight CTBN is chemically bonded through terminal ester groups with 2,2′-bis(4-hydrophenyl) propane units during manufacture. Pham S., Polymer 1995, 36:3279-3285. This toughened VE resin system exhibits moderate fracture toughness but also includes a large amount of styrene, namely, 40% by weight.
Triglycerides are found in oils, such as soybean oil, linseed oil, etc. Soybean oil, as an example, is a renewable resource which contains different kinds of unsaturated fatty acids and saturated fatty acids with varying carbon chain lengths. Three unsaturated fatty acids with varying functionalities are connected by a glycerol center. The average unsaturation degree is 4.6. Epoxidized soybean oil (ESO) is a type of functionalized triglyceride. ESO has been used as a composite (Thielemans W., et al., Journal of Applied Polymer Science, 2002, 83:323-331 and Lu J., et al., Polymer, 2005, 46:71-80), a lubricant, a plasticizer, and a thermal stabilizer (Lathi P. S., Applied Catalysis B: Environmental, 2007, 69:207-212, Demertzis P. G., et al., European Polymer Journal, 1991, 27(3):231-235 and Liu P., et al., Polymer Degradation and Stability, 2007, 92:503-508). Using ESO to toughen epoxy resins is also known (Park S. J., et al., Materials Science and Engineering A, 2004, 374:109-114, Ratna D., Journal of Adhesion Science and Technology, 2000, 14(1):15-25 and Miyagawa H., et al., Polymer Engineering and Science, 2005, 45(4):487-495.
U.S. Pat. No. 6,121,398 (Wool et al.) discloses high modulus polymers and composites that are derived from plant oils. This patent includes an extensive discussion of the various types and uses of triglycerides obtained from natural sources such as plant oils. This patent discloses functionalized triglycerides that are polymerizable and their use to produce high modulus polymers. The functionalized triglycerides may be produced via a number of different chemical synthesis routes. For example, epoxidized triglyerides may be produced and converted to resilient rubbers by control of the molecular weight and cross-link density. The resultant rubbers can be used as rubber toughening agents in rigid composites. In the examples of this patent, acrylated base resins are prepared by reacting the epoxidized triglycerides with acrylic materials such as acrylic acid. In Example 34 of this patent, 8 grams of acrylated epoxidized triglyceride was mixed with a range of 2-8 grams of Dow Derakane™ 411 vinyl ester resin and 0.2 grams of USP 245 free radical initiator, purged with nitrogen and heated for one hour at 90° C. and an additional hour at 110° C. to provide a rigid thermoset resin. The thermoset resins prepared by this method are said to have properties similar to commercial bisphenol-A vinyl ester resins, i.e. a tensile modulus of about 3 GPa, an elongation at break of 10%, a flexural modulus of about 2.8 GPa and a heat deflection temperature of 75° C. Other functionalized triglycerides are described in U.S. Pat. No. 6,825,242 and U.S. patent application publication nos: US 2003/0139489 and US 2009/0275715.
Vinyl ester and unsaturated polyester resins contain styrene, a hazardous air pollutant and carcinogen. The styrene in these resins is regulated under the EPA Reinforced Composites NESHAP. Most commercial solutions simply reduce the styrene content in the resin (˜33 wt % styrene) making the resin barely acceptable for composite manufacture applications. In addition, reducing the styrene content significantly reduces the toughness of these resins. Recent solutions, such as methacrylated fatty acids show promise in reducing the styrene content, but result in a significant drop in glass transition temperature for the resin.
Accordingly, a need exists for new types of vinyl ester toughening agents to meet current requirements for toughening vinyl ester resin systems.